1. Field of the Invention
The present invention relates generally to a measuring system. In particular, the system and method of the invention are directed to a position measuring system incorporating a laser tracker and an active target. The active target of the position measuring system is capable of 360 degree rotational tracking range to maintain alignment with the tracker laser beam at all times. The position measuring system offers unique advantages in manufacturing processes wherein the position of the machine or machine tool must be precisely determined and any positional errors due to changes in the temperature, mechanical alignment or the like can be detected and corrected.
2. Related Art
Precision measuring systems have a wide variety of applications. In three-dimensional metrology applications such as robotics, laser pointing, laser tracking interferometry and laser radar measurement systems, accurate positioning and orientation is often required of a robot, a device coupled to the robot, a machine tool or other mechanical positioning devices, including various types of computer numerical control devices. To achieve a high degree of precision, a position measuring system can be used. Such a system typically uses a laser beam interferometer and a retroreflector as a target, wherein the target is illuminated by the interferometer and the reflected light detected and analyzed to determine the position and/or orientation of the retro-reflective target. As an example, a retro-reflective target can be mounted upon the end effector of a robot and the system can monitor the position and orientation of the end effector in real-time while providing accuracy, speed and measurement data.
Since the position of a retro-reflective target can be precisely determined, retro-reflective targets are widely employed during the manufacture of parts demanding high precision, such as parts fabricated for the aerospace and automobile industries. In these applications, retro-reflective targets can be mounted upon machines, such as robots or other machine tools, utilized during the manufacture of precision parts such that the position of the machine can be precisely determined and any positional errors, such as positional errors due to changes in the temperature, mechanical alignment or the like between the tool and the part, can be detected and corrected.
When using large machine tools in large-volume work spaces many factors can conspire to reduce the accuracy of the tool throughout its working volume. Factors like machine load, force of cutting, foundational deviations, and simple wear over time make the machine less accurate.
As an illustrative example, in the manufacture of large-scale precision parts, such as those produced in the aerospace and automotive industries, large-scale machine tools are used to process large-scale parts. The manufacturing processes are performed in large volumetric work spaces, in which the side dimensions of the work platforms can exceed 200 meters. The machine tools exhibit dimensional positioning errors that are difficult to minimize. The primary sources of these positioning errors are the expansion and contraction of the machine structure and the workpiece due to temperature changes in the work space during machining, and mechanical misalignments of and between individual axes of the machine. These positional errors need to be monitored, detected and corrected to produce consistently high-quality precision parts.
Most existing solutions to these problems involve highly specialized measuring equipment and substantial machine downtime for measurements. Traditional methods often taken several days to perform and require repeated set-up changes of the measuring equipment. Accurate results are almost impossible due to temperature fluctuations during this, long process. The common three-axis machines have 21 error parameters in addition to the errors introduced with the machine spindle. Machine calibration techniques measures some or all of these 21 error parameters, then makes physical or software adjustments to the parameters which are out of tolerance. Many times even with these complicated techniques only a fraction of the 21 error parameters in a given machine volume are actually measured. Since so many of the possible machine errors are not accounted for, the final results of this type of machine error compensation are not satisfactory for improved production quality and speeds. With the machine tool being idle for several days during the measurement process, it is not uncommon for tens to hundreds of thousands of production dollars to be lost due to machine downtime.
Improving the accuracy of a machine, automated tool, or robot so that parts are manufactured closer to design specifications involves augmenting the machine control with an independent, higher-accuracy position measurement system to correct for machine-related and factory-induced errors. The independent measuring system identifies the true position of the end effector when the machine stops prior to machining. This machine control augmentation process to correct for the machine and factory-induced errors can be referred to as volumetric error compensation of the machine tool.
Ideally retroreflectors used in these applications should have an unlimited field of view such that the retroreflector can receive and reflect light that impinges upon the retroreflector from any direction. Conventional retroreflectors have a limited field of view known as an acceptance angle or angular working range. Light received by a retroreflector within the acceptance angle is reflected by the retroreflector. However, light outside of the acceptance angle is not reflected and, therefore, cannot be utilized to determine the position of the retroreflector. Thus, the acceptance angle of a retroreflector restricts the position and orientation of the retroreflector relative to the light source. This limitation is particularly disadvantageous in applications in which the retroreflector is mounted upon a machine, such as a robot or other machine tool, that can move in multiple directions and about multiple axes relative to the light source and may frequently be positioned such that the retroreflector does not face the light source, thereby preventing the light emitted by the light source from falling within the acceptance angle of the retroreflector. Without adding additional light sources and/or additional retroreflectors which would, in turn, increase the cost and complexity of the precision measuring system, the position of the machine can therefore not be determined in instances in which the retroreflector does not face the light source.
One commonly-used retroreflector is a trihedral prism reflector that is frequently referred to as a solid corner cube retroreflector. While trihedral prisms are relatively inexpensive and are fairly accurate with respect to the parallelism of the incident and reflected beams, they are limited to an acceptance angle of about +/−20° and still be able to provide good accuracies. With this type of prismatic retroreflector, use of a different type of materials can increase the acceptance angle up to +/−50°. However, accuracies are poor.
Another type of retroreflector is a hollow corner cube retroreflector that is constructed of three mutually orthogonal mirrors. Although the lateral displacement between the incident and reflected beams does not vary as a function of the incidence angle, a hollow corner cube retroreflector is more expensive than a comparable trihedral prism reflector. A hollow corner cube retroreflector typically has an acceptance angle of about +/−20°, which is comparable to that of the trihedral prism reflector.
A third type of retroreflector used is a cat eye in which several hemispherical lenses are bonded to form a single optical element. While a cat eye has a larger acceptance angle, about +/−60°, a cat eye is significantly more expensive than a trihedral prism retroreflector or a hollow corner cube retroreflector. Although a cat eye has a much greater acceptance angle than the other two types of retroreflectors, the acceptance angle is still insufficient in many situations, particularly in many high precision manufacturing operations in which the retroreflector will be mounted upon the end effector of a robot or other machine tool that will assume many different positions during the manufacturing process. Moreover, the material from which the cat eye retroreflector is made affects its refractive characteristics. Thus, the cat eye does not work well with a laser beam having different frequency wavelengths. In a typical interferometer tracking laser, visible light is used with the interferometer while infrared light is used for the absolute distance measurement (ADM).
Thus, although a variety of retroreflectors are available, these conventional retroreflectors do not have acceptance angles that are sufficiently large and continuous as required by some applications. Preferably, retroreflectors which are mounted upon the end effector of a robot or other machine tool should have an extremely large acceptance angle since the retroreflectors will be moved through a wide range of positions during typical machining operations. As such, there remains a need for a retroreflector having a much larger acceptance angle than conventional retroreflectors, while still being capable of being economically manufactured and deployed.
Volumetric compensation of large machine tools using a laser tracker-based volumetric error compensation (VEC) system requires that the tracker track to a well-defined reference point on the machine during the measurement. This point usually is on the axis of rotation of the machine spindle or tool holder. Traditionally, a spherical mounted retroreflector (SMR) optical target, which is a standard tracking target for a laser tracker system, is attached to the spindle and aligned to the axis of rotation to establish the measurement reference point. A SMR employing any of the above-described retroreflectors, however, has a limited angular working range, or acceptance angle, from ±20 to ±25 degrees. Although a cat eye retroreflector provides a greater angular working range, it still is insufficient for many applications. A typical VEC measurement requires 360 degree angular tracking range. This requires that the SMR retroreflector face the tracker laser beam at all times during the measurement. This is usually achieved by repeatedly rotating the SMR manually every 20 degrees or so during the measurement session. This human intervention greatly slows down the measurement speed and increases the uncertainties of the overall measurement quality. Operator's safety is also a great concern.
Attempts have been made to overcome some of the foregoing issues. In one prior-art attempt, Greenwood (U.S. Pat. Nos. 6,420,694 and 6,392,222) describes a steerable retro-reflective system in which leakage laser light passing through a retroreflector and impinging on an optical detector provide signals used by control positioners to steer the retroreflector and keep it aligned with the tracker beam. This system is stated to have an acceptance angle of 320 degrees or more. However, a relatively-wide 40 degree angle can not be covered.
Corby (U.S. Pat. No. 5,633,716) describes a retroreflector assembly in which a misalignment detector is used to maintain alignment of the reflected laser beam within the acceptance angle of the tracker detector. In one embodiment, the misalignment detector uses light from a source located on the tracker impinging on a photodetector to generate corrective signals for azimuth and elevation actuators to control positioning of the reflector. In another embodiment, a portion of the incident beam is provided to the photodetector. In both embodiments, the reflector target is mounted on a cradle, and the misalignment detector is mounted in a fixed orientation relative to the reflector.
Merry et al. (U.S. Pat. No. 4,621,926) describes an interferometer control system using a target capable of pivoting about a horizontal shaft. Motors on the tool holder itself reciprocate and rotate the tool holder and the attached target along the z-axis. However, complete rotation of the target about the z-axis is not possible since anything more than a nominal rotation in either direction will break the line-of-sight between the target and the tracking laser beams. In the tracker head, the output signal from a position-sensitive detector is used to control the position of the tracking mirror such that the output laser beam strikes the center of the retroreflector.
Thus, for precision measuring systems in which the retroreflectors will be moved through a wide range of positions during typical machining operations, it is desirable for the retroreflector to have an extremely large acceptance angle. In order to have the capability of precisely determining the position of a machine as the machine assumes a variety of positions such that the machine can be driven to compensate for positional inaccuracies and to form parts with precise dimensions, a need remains for a precision measuring system having an active target that provides a full, unlimited 360 degree angular working range.